On 14/09/2017 20:14, T i m wrote:
Hi all and knowing there are some scientists / mathematicians walking
amongst us ... ;-)

I'm trying to see if there is a (rough even) way of measuring the
Depth Of Discharge voltage / cutoff point versus current load value
for a specific lead acid battery.

The manufacturers have provided me the following values (@ 50% DOD).

11.75V @ 93A (12.4V initial)
12.05V @ 18.2A (12.7V initial)
12.10V @ 4.65A (12.75V initial)

(The general chart starts at the values marked 'initial' so that would
be for those currents but at 0% DOD but I'm not sure if that is
relevant).

Now, it's obviously not a straight line (Peukert's law), but can you
extrapolate a graph (or create a formula that would be more useful for
my project) from just 3 points please?

Maybe Peukert's law itself would give those with a brain wired
differently than mine the answer? ;-)

https://en.wikipedia.org/wiki/Peukert%27s_law#Formula

Cheers, T i m

p.s. The nearest *I* could get to an answer would be some graph paper
and a Flexicurve. ;-)


Probably no simple answer because it all depends on the shape of the curve.

I'm not familiar with that particular "law" (I use the term advisedly)
but exponentials are buggers to deal with (especially when, as in this
case, it is obviously only an approximation: exponentials are fine for
radioactive decay, but it will certainly have limits in this case).

I think my approach would be to try to collect some data for your
specific battery, and try to work with that. Quite possibly with a
flexicurve, or with some sort of polynomial fit if there was more data
available.

Sometimes you can over-think a problem, you have to work with the data
and tools available.

As an example, compare the different "lines" of a Spitfire or a Vanwall
with the various front wings of current F1 cars. Modern stuff has the
benefit of serious CFD and wing-tunnel testing. And it is justified in
that context.